In optical wavelength division multiplexed (WDM) communication systems, a single optical waveguide simultaneously carries many different communication channels in light of different wavelengths. In general, each communication channel is assigned a nominal centre wavelength, and the channel spacing, or separation, is defined for the network. The closer the channel spacing, the greater the number of channels that may be transmitted over an optical fiber of the network. The International Telecommunications Union has proposed Dense WDM (DWDM) network standards with optical signal channels having a frequency separations of 25, 50, and 100 GHz (equivalent to a wavelength separation of about 0.2, 0.4 and 0.8 nm, respectively). Lower frequency separations are envisioned.
Accordingly, the performance requirements for DWDM networks (such as those for bandwidth, cross talk, polarization dependent loss, polarization mode dispersion, and insertion loss) are becoming more stringent. In addition to the above problems, selective routing is difficult in DWDM communication systems because of the limitations introduced by conventional optical switches.
Conventional optical switches are typically based on optical-electrical-optical (OEO) technologies. In an OEO scheme, the optical signal is transduced into an electrical signal, the signal is switched electrically, and is reconverted back into a new optical beam. Unfortunately, the OEO conversion is limited by the processing speed of the available electronics. Furthermore, OEO devices are dependent on wavelength, modulation format, and modulation frequency.
More recently, there as been increased interest in all-optical switching, in which one or more wavelengths are selectively switched without the need to convert the optical signals to an electronic signal. Micro-electro-mechanical systems (MEMS) have played an important part in all-optical switching since these miniature actuators can be designed to simultaneously switch spatially resolved portions of the optical signal independently from each other. Furthermore, MEMS devices can be designed to be compact, have a low power consumption, and can be mass produced to produce a low cost switch. Liquid crystal (LC) modulators have played an important role in all-optical switching for similar reasons.
In many prior art switches using MEMS or LC modulators, the switch includes a dispersive element to spatially separate the multiplexed beam of light into individual communication channels, which are independently modified by the modulator. The dispersive element is typically a reflective or transmissive diffraction grating used in either a single pass or double pass configuration. For example, in the single pass configuration a first diffraction grating performs the demultiplexing while a second diffraction grating performs the multiplexing. In the double pass configuration, a single diffraction grating is combined with a reflector to provide demultiplexing in a first pass therethrough and multiplexing in the second pass therethrough.
However, since each communication channel is generally incident on a separate element or pixel of the MEMS or LC modulator, a small portion of the optical signal is lost due to the gaps between discrete pixels. In particular, the opaque gaps between pixels in LC modulators and/or the space between reflectors in MEMS modulators removes (e.g., blocks) a portion of the dispersed spectrum. This creates a spectral ripple in either amplitude or phase of the optical signal. When the transmission signal passes through more than one of these switches, the spectral ripple accumulates and causes significant transmission errors. For example, a significant narrowing of bandwidth is observed.
In an attempt to obviate the bandwidth narrowing associated with cascading multiple switch devices, U.S. Pat. Nos. 6,389,188 and 6,320,996 to Scobey et al., incorporated herein by reference, propose an all-optical switch that only wavelength de-multiplexes/multiplexes the optical channels to be switched, with minimal signal degradation to the express channels. However, the proposed switch is limited by the wavelength range of the filter used therein, and cannot be reconfigured without physically modifying the device. In other words, this switch is unable to switch a variable number of non-consecutive channels.
In U.S. Pat. No. 5,943,158 to Ford et al., incorporated herein by reference, there is disclosed an attenuator based on a mechanical anti-reflection switch (MARS) that provides a continuous, uniform optical surface. However, this device is not suitable for use in a wavelength selective switch since it is limited by the mechanical properties of the continuous membrane. More specifically, the non-discrete properties of the mechanical membrane result in a coupling between the controls exercised by nearby electrodes, and limits the achievable spatial and hence wavelength resolution.